A standard press of the type used in the continuous production of fiberboard, particleboard, and the like has a rigid press frame having vertically spaced upper and lower parts defining a press gap. Upper and lower belts are spanned in the respective press parts between respective upstream and downstream rollers, at least one of which is driven to advance confronting upper and lower stretches of the lower and upper belts longitudinally through the press. Upper and lower press plates bear, typically by some sort of roller arrangement, on the lower and upper surfaces of the upper and lower stretches of the lower and upper belts. The lower press plate is fixed. The upper plate is vertically movable by means of a double-acting hydraulic actuator system.
Normally a plurality-of hydraulic cylinders (see U.S. Pat. No. 3,545,370) are provided on each frame bearing downward on the upper plate. Some movement of the upper plate, which may be formed of a single steel plate or of several joined-together plates, is possible since as the incoming mat is compressed and heated it resists compression differently. Thus it is known to provide the press with a sensor system for determining the press gap. Such sensors generate outputs that can be used to shut down the system when, for example, there is an interruption of the incoming workpiece so that the press bands could be pushed directly against each other, which would damage them.
In a known press described in commonly owned U.S. Pat. No. 5,112,431 of Gerhardt the press gap is set with a system of wedges They serve both to set the distance as well as to prevent excessive bending of the upper press plate. In this latter safety function they define the minimum gap that the press can have. It is also known from German patent 2,451,894 to use hydraulic cylinders having built-in stops which can be set up as wedges or just installed right into the cylinders. Such an arrangement limits the flexibility of the press with respect to the various working conditions it must meet, in particular the different workpieces the press must deal with.